According to Wikipedia, the UK's mains voltage is 230V +/- 10%. For those of us who wish to divert surplus PV power for heating water, any change in mains voltage seems likely to cause complications. If the voltage goes down by a few percent, then in order to make the load consume the correct amount of power it would be necessary to increase the requested power setting by a similar amount, and vici versa.
The effects of voltage change can probably be minimised by measuring power only at the grid supply point. As the desired condition is just a few tens of Watts of exported power, I think this target should be relatively immune from such changes. By measuring at just one place, negative feedback is also built-in from the outset, thereby allowing the control system to home in on the desired value.
I wonder whether this topic has been aired already?
Re: Effect of variable mains voltage
Slight error, you read the wrong bit of Wikipedia - it's 230 V + 10% - 6%. Historically, the UK mains was 240 V +/- 6%. Due to the cost of replacing a huge proportion of the transformers in the distribution network, the UK supply remains unchanged and centred on 240 V, and it's very unlikely to go below 225 V! The tolerances have been fiddled so that it more-or-less complies with the rest of Europe standard.
Otherwise, to address your main point, power (for a non-controlled appliance - I exclude anything with a regulated power supply) will change by roughly twice the voltage change percentage-wise, but the solar inverter and appliances with regulated power supplies are likely to maintain constant power. You can't actually say whether the overall current will go up, down or remain the same - it will depend on what loads you have connected. So my immediate thought is that you are going to have to rely on the feedback loop and hope it has a long enough time constant, or an integral term, to remain stable. My second thought is you might need to take current as well as power into account if you want to be really precise, but I haven't thought that one through.